High-side switches can be used to drive a variety of loads, and therefore can be used in a number of different applications. Typical systems and methods for driving a high-side switch utilize a charge-pump. A charge-pump is a DC to DC converter that uses capacitors as energy-storage elements to create either a higher- or lower-voltage power source. In regards to high-side switches, the charge-pump is relied on to supply other circuit components (such as amplifiers) in addition to supplying a DC current for driving the high-side switch. This method necessitates the use of large capacitors within the charge-pump to supply DC load currents. Large capacitors can take up valuable surface area if an on-chip integrated solution is required. To solve this problem, some systems implement external capacitors for supplying the DC current. While this reduces the required surface area of the chip, extra pins are then included to connect the external capacitors. Using a charge-pump design for driving a high-side switch is not conducive to situations that require a reduced chip size or situations that are cost sensitive and therefore require a reduced number of pins. Additionally, using a charge-pump design is not conducive to situations that require as few external components as possible, such as external capacitors, because the external components also add to the overall bill of materials (BOM) and cost.
Generally speaking, high-side switches include three main elements: a pass element, a gate-control block, and an input logic block. The pass element is usually a transistor which is typically a metal-oxide-semiconductor field-effect transistor (MOSFET) or a laterally diffused metal oxide semiconductor transistor (LDMOS). An LDMOS transistor is considered to be a type of MOSFET. The pass element operates in the linear region to pass the current from a power source to a load. The gate-control block provides a voltage to the gate of the pass element to switch it “on” or “off.” The input logic block interprets the on/off signal and triggers the gate control block to switch the pass element “on” or “off.”
In electronics, slew rate is defined as the change of voltage per unit of time. Exceeding a circuit's slew rate can cause signal distortion. Also, exceeding the slew rate can cause an increased amount of electromagnetic emissions (EME), thereby violating electromagnetic compatibility (EMC) standards, and potentially disturbing other electronic devices. Accordingly, the slew rate can place significant limitations on the operation of a corresponding circuit. Adding current limiters can provide some control over slew rate, but this solution still requires the use of a large charge pump.
FIG. 1 is a circuit-level schematic of a known system and method for driving a high-side switch with additional current limiters. As shown, a charge-pump 2 is connected to a current controller 4. The current controller 4 includes an amplifier 6 and a transistor 10. Here, the transistor 10 used is a p-channel metal-oxide semiconductor (pMOS). The current controller 4 is supplied by the charge-pump 2 and controls an output based on the voltage difference resulting from resistors 12, 24. The positive rail of the amplifier 6 is powered by the charge-pump 2 and the negative rail of the amplifier 6 is powered by a supply voltage 8. A current sensing resistor 12 is connected between the charge-pump 2 and the amplifier 6. A current-sensing FET 14 is connected between the amplifier 6 and an output pin 18. A high-side switch FET 16 has the drain side connected to the charge-pump 2, the gate side is connected to the output of the amplifier 6, via transistor 10, and the source side is connected to the output pin 18. The output pin 18 is used to connect the system to a circuit load 20. Additionally, a resistor 32 is connected between the gate side and the source side of the FET 16. The circuit further includes a clock generator 22. A resistor 24 is connected between the charge-pump 2 and the amplifier 6. The circuit further includes a load reference 26. Also shown, a FET 28 is connected in series with a resistor 30 for when the high-side switch FET 16 is turned “off.” Current limiters 34 are used to provide some control over the slew rate of the high-side switch FET 16.
Still referring to FIG. 1, the charge-pump 2 needs to deliver a significant output current due to its connection to the amplifier 6 and the high-side switch FET 16. Quickly charging the gate of the high-side switch to the desired voltage VGS with the current controller 4 draws a significant amount of current from the charge-pump 2. Therefore, the charge-pump 2 includes relatively large capacitors, making it difficult to integrate the circuit of FIG. 1 onto a single chip. Large integrated capacitors increase the silicon die size and thereby product cost. Should the large capacitors be externally located, additional pins become necessary, and any external capacitors may increase the BOM cost, therefore increasing the cost of the overall system. Controlling the slew rate of the high-side switch FET 16 with the current limiters 34 results in the need for a large charge-pump 2.
Therefore what is needed is an improved system and method for controlling the slew rate of a high-side switch.